A thermoplastic injection mold is formed, among other components, by two shaping plates, one referred to as cavity plate and another referred to as die plate, as well as a set of ejector plates where several ejector pins are housed, which will serve for ejecting the injected part.
An injection mold can have one or several shaping cavities, wherein at the time of injection the duly melted plastic material is introduced under pressure through an injection nozzle or point, until completely filling of each of the cavities.
Each mold incorporates a refrigeration circuit made up of machined boreholes in the shaping plates which serve to cool the injected plastic and set the part.
The ejector pins located in through housings structurally configured in the die plate are activated through the ejector plates, being responsible for extracting the part from the cavity once the plastic has set.
This housing does not have a uniform diameter along its entire length, instead the ejector pins are really tightly fitted, with a very narrow clearance marked by standard regulation, in the portion nearest the cavity, whereas at rest the housing has a greater diameter to prevent unnecessary friction with the ejector pins.
During the injection process the mold is closed and therefore the cavity is full of air which is gradually compressed as the plastic material is injected, reducing its volume and increasing its pressure, which complicates filling it with the plastic material.
This entire process requires, in many cases, an increase in temperature of the plastic mass as well as a greater injection pressure so that the entire mold cavity can be filled, mainly in areas with a difficult configuration, which entails a longing cooling time before being able to demold the duly set part.
In many cases, due to the temperature and pressure excess applied to the plastic material to fill the cavity, internal stresses are produced in the part itself such that the dimensional stability is lost, negatively affecting the final quality of the product.
To solve these problems some solutions are known which allow the exit of air but at the same time prevent the exit of the injected material.
These solutions include the creation of air removal conduits located in the perimeter of the cavity, usually in the area furthest from the injection point. The air mass also escapes the cavity through the small clearance between the ejector pins and their housings.
The arrangement of a plugging element in the access opening of the ejector conduit, enabling the exit of air, preventing the exit of the injected material, is also known.
This plugging element is in some cases made up of an air permeable filter and in other cases by a cap or closing element which tends to remain in an open position by action of a spring. This closing element allows removing air and closes automatically when the pressure of the injected material overcomes the pushing of the spring, thus preventing the exit of said injected material through the removal conduit, although this solution is not applicable with all materials.
These molds have several drawbacks in relation to the air removal means, the most noteworthy being that the injected material acts as a plunger pushing the air through the removal conduit, which requires greater injection pressure; and that the injected material cools more readily since it is in contact with the air, and therefore it is necessary to inject said material at a greater temperature with the subsequent energy cost. In order to achieve a greater effectiveness in removing the air it is necessary for the plugging element be located at the end opposite the injection nozzle, the molten material reaching this area with less fluidity. It is necessary to invest more energy and production costs to obtain the part during the process.
Furthermore, these problems also affect the quality of the obtained part since air bubbles are trapped in the molded parts and clearances are formed in certain configurations. The inadequate extraction of air produces grooves in the outer faces of the injected parts and during the filling of the mold the injected material is subjected to stresses which cause imperfections and deformations in the obtained parts.